608 MANUAL OF PHYSIOLOGY. 



in it can be appreciated, since the more ample the vibration the 

 more marked the motion, and, therefore, the more intense the 

 stimulation of the nerve terminals. What we call the loudness 

 of a sound, then, simply means greater or less intensity of stimula- 

 tion of the nerve. 



The comprehension of the perception of differences of pitch pre- 

 sents greater difficulty. As already mentioned, this depends on 

 the rate or period of vibrations. We know that most bodies 

 capable of producing sound vibrations have a proper tone, i.e., 

 that which they produce when struck. When the proper tone of 

 a body capable of vibrating is sounded in its immediate neighbor- 

 hood, this is also set vibrating through the medium of the air. If 

 a clear tone be sung loudly over the strings of a piano a kind of 

 sympathetic echo will be heard to come from the cords, on account 

 of the strings corresponding to the notes sounded being thrown 

 into sympathetic vibrations. Now, in the basilar membrane we 

 have practically a series of strings of different length since the 

 membrane gets wider as it passes from below upwards to the 

 summit of the cochlea and therefore a great variety of proper 

 tones. With a high note, then, a fibre of one part of the mem- 

 brane will readily fall into vibration, and with a low note a fibre 

 of another part. Different nerve fibrils are in relation to these 

 different parts, and thus we may conclude that tones of different 

 pitches stimulate distinct nerve-terminals, and are conveyed to 

 the brain by separate nerve-channels. Impulses arriving at cer- 

 tain brain-cells then give rise to the idea of high tones, and im- 

 pulses coming to others cause the impression of low tones. There are 

 about a sufficient number of fibres in the basilar membrane for 

 all the notes we can hear, viz., from about 30 to 4000 waves in the 

 second. 



The rods of Corti cannot be the vibrating agents, because they 

 are too few in number, and they are absent in birds, which appre- 

 ciate and reproduce various notes. Further, the rods are not 

 elastic, and therefore not well suited to vibrate. It may therefore 

 be concluded that they only act as levers which convey the vibra- 

 tions of the fibres of the basilar membrane to the nerve-endings 

 in the auditory cells. 



